Anaerobic Digester Supernatant Research Articles

Nutrient recovery from anaerobic digester supernatant using a fluidized-bed reactor

This study investigated nutrient recovery by struvite precipitation from anaerobic digester supernatant containing nitrogen and phosphorus at high concentrations using a fluidized-bed reactor. In the first phase, hydraulic retention time (HRT) in the system was reduced from 15 min to 2.5 min at molar Mg2+/NH4+/PO43− ratio of 1.5/16.6/1. No significant differences were detected and struvite could be precipitated at high performances even at an HRT of 2.5 min. In the second stage, the impact of Mg/P molar ratio on struvite precipitation performance was evaluated at the molar ratios of 1/1, 1.5/1, 2/1 and 2.5 min HRT. The highest NH4-N and PO4-P precipitation performances were around 38 % and 99.7 %, respectively, at an Mg/P molar ratio of 2. In the last stage, the impact of volatile fatty acid presence on the system performance was investigated by adding 1000 mg/L acetate to the synthetic wastewater at an Mg/P molar ratio of 1.5/1. Observing above 99 % PO4-P removal efficiency both in the presence and absence of acetate confirmed that VFA may not have adverse impact on struvite precipitation performance. According to XRD results, struvite precipitates had a crystal structure with only a small amount of impurities.

Environmental and economic evaluation of implementing membrane technologies and struvite crystallisation to recover nutrients from anaerobic digestion supernatant

The present study investigates the environmental and economic feasibility of implementing membrane technologies and struvite crystallisation (SC) for nutrient recovery from the anaerobic digestion supernatant. To this end, one scenario combining partial-nitritation/Anammox and SC was compared with three scenarios combining membrane technologies and SC. The combination of ultrafiltration, SC and liquid–liquid membrane contactor (LLMC) was the less environmentally impactful scenario. SC and LLMC were the most important environmental and economic contributors in those scenarios using membrane technologies. The economic evaluation illustrated that combining ultrafiltration, SC and LLMC (with or without reverse osmosis pre-concentration) featured the lowest net cost. The sensitivity analysis highlighted that the consumption of chemicals for nutrient recovery and the ammonium sulphate recovered had a large impact on environmental and economic balances. Overall, these results demonstrate that implementing membrane technologies and SC for nutrient recovery can improve the economic and environmental implications of future municipal wastewater treatment plants.

Sustainable Removal of Ammonia from the Anaerobic Digester Supernatant Line Using a Prussian Blue Analogue (PBA) Composite Adsorbent

This paper reports on the physico-chemical removal of NH4+ from the supernatant line in municipal wastewater treatment plants (WWTPs), using zinc-hexa-cyano-ferrate (ZnHCF) beads. The work is divided into three parts: First, the characteristics of three (Zn-, Co-, Ni-) types of HCF beads were determined, with a finding that ZnHCF was the most suitable for the purpose of this work. Second, synthetic and actual supernatant wastewater was passed through a ZnHCF column for many cycles until apparent steady-state results were attained. Due to the very high affinity of the beads toward NH4+ and the much lower affinity toward competing cations, the same regeneration solution could be used for many cycles (20 cycles in this work) without affecting the following adsorption breakthrough curve efficiency and the operational capacity, which was >88% at the end of all adsorption steps. Finally, a cost analysis was performed, revealing that the cost of removing ~500 mg/L of ammonia from the supernatant line is ~$0.02 per m3 of raw wastewater flowing into the plant if the ammonia is recaptured and sold as NH4Cl. This may be cost-effective when the WWTP receives a higher-than-planned load, and an incentive exists for alleviating the ammonia load on the oxidation reactor.

Biogas purification and ammonia load reduction in sewage treatment by two-stage down-flow hanging sponge reactor

In this study, a two-stage down-flow hanging sponge (TSDHS) reactor was used as biotrickling filter for biogas desulfurization by utilizing the anaerobic digester supernatant (ADS) of sewage sludge of an activated sludge process (ASP). The reactor comprises a closed-type first-stage down-flow hanging sponge (1st DHS) and an open-type second-stage down-flow hanging sponge (2nd DHS) reactors. In the 1st DHS, hydrogen sulfide in biogas was dissolved into the ADS, and then it was oxidized into elemental sulfur and sulfate by microbe using dissolved oxygen and nitrite in the ADS. More than 99.9 % of hydrogen sulfide was removed within 400 s of empty bed residence time, and >50 % of removed hydrogen sulfide was oxidized into elemental sulfur and accumulated at the surface of the sponge carrier in the 1st DHS. The 1st DHS effluent was fed into the 2nd DHS for nitrogen removal via nitrification and sulfur-based denitrification with the recirculation of the 2nd DHS effluent under nonaeration condition. In the 2nd DHS, 36.8 % of ammonia and 5.3 % of total inorganic nitrogen were removed. Sulfurimonas and Halothiobacillus were increased and contributed to the sulfur-based denitrification as well as the accumulation of elemental sulfur in the 1st DHS, respectively. In the 2nd DHS, Nitrosococcus, Nitrobacter, and Sulfuritalea were considered as the contributors of nitrogen removal via nitrification and sulfur-based denitrification. Further, this study shows that a TSDHS reactor can achieve not only desulfurization of biogas in the 1st DHS but also a 3.5 %–15 % reduction of the ammonia load in the 2nd DHS by effective utilization of the ADS during sewage treatment, assuming that the ADS is returned to the ASP.

Effect of calcium peroxide pretreatment on anaerobic digestion of primary and secondary sludge of A2/O process

In order to evaluate comprehensively the impact of calcium peroxide (CP) on anaerobic digestion (AD) of primary sludge (P-sludge) and secondary sludge (S-sludge) of anaerobic/anoxic/aerobic process, a series of batch experiments were conducted in the lab scale. Results showed that CP pretreatment could kill Fecal Coliform completely in 72.0 h and promote the solubilization of S-sludge more significantly than P-sludge. Compared with the control, the maximum CH4 production potential and the maximum specific CH4 production rate of P-sludge AD only increased by 11.2 % and 6.10 %, respectively, but those of S-sludge AD increased by 40.7 % and 58.4 % after the same pretreatment, indicating that CP pretreatment was more conducive to methane production of S-sludge. Moreover, the increase of alkalinity and initial acetate concentration of S-sludge was greater than that of P-sludge after pretreatment according to the process analysis of hydrolysis. Notably, CP pretreatment reduced the PO43−-P concentration and chromaticity in the AD supernatant of both sludges, which was beneficial to the subsequent treatment of biogas slurry. In addition, CP pretreatment increased the content of total nutrient and decreased the content of harmful heavy metals in both biogas residues. Conclusively, compared with P-sludge, CP pretreatment is more suitable for S-sludge AD. This work firstly provides a comprehensive evaluation of CP pretreatment to waste sludge produced in anaerobic/anoxic/aerobic process of wastewater treatment plant (WWTP), which is of great significance in enriching the pretreatment technology of waste sludge and promoting the application of CP pretreatment in WWTP.

Recovery of struvite from wastewaters as an eco-friendly fertilizer: Review of the art and perspective for a sustainable agriculture practice in India

The limited rock phosphate deposits present in the earth’s crust are being used at a fast rate and therefore phosphate fertilizers use will be restricted in the future. Hence, there is an urgent need of conserving the use of phosphorous (P) and recover/reuse of P from wastewaters to the maximum extent. Struvite (MgNH4PO4) is a crystalline mineral composed of magnesium, ammonium, and phosphate, and is considered as an adequate source of nutrients especially P. Struvite can be used as raw material for fertilizer production or as slow-release fertilizer. Recently, there were a lot of researches carried out to explore the possibility of recovering the struvite from wastewater streams. This paper reviews the potential sources of P in wastewater streams and the techniques of struvite recovery available in the literature for practical applications. Anaerobic digester supernatant, urine from livestock and humans are identified as potential wastewater sources of P in India. Continuous stirred tank reactor technique can be easily adopted to recover P, which is known to recover 80–90% P in wastewater. Currently, India is the second largest consumer of P due to the low P in soil and depends on imported sources for meeting her demand on fertilizers. Therefore, the work presented in this review would help India to formulate suitable policy guidelines to meet a part of the P demand by recovering struvite from wastewater streams and apply in agricultural field as an eco-friendly fertilizer. Such an effort significantly contributes to promote circular economy and sustainable agriculture.

Optimizing granular anammox retention via hydrocycloning during two-stage deammonification of high-solid sludge anaerobic digester supernatant

High-solid sludge anaerobic digestion leads to increased organic matters in digester supernatant, which promotes heterotrophic competition and reduces anaerobic ammonium oxidation (anammox) retention. This research demonstrated effective anammox retention by hydrocycloning during a two-stage deammonification. Anammox retention was evaluated by dividing large (>0.425 mm), medium (0.25 to 0.425 mm), and small (<0.25 mm) aggregate fractions via a sieve, whereby Candidatus Kuenenia abundance in each size aggregate was measured to be 16.8%, 5.0%, and 0.9% respectively. After hydrocyclone separation, large particles took up only 1.7% to 2.7% in the overflow discharge (upper discharge from the reactor), whereas its initial proportion was 19.4%, indicating limited anammox loss. The volume ratio change of large particles to total aggregates was defined for particle breakdown evaluation. Breakdown (23% to 32% large particles) occurred mainly during pumping (influenced by pump frequency and type), while little happened in the hydrocyclone. This research provided methods to use a sieve to evaluate anammox retention by hydrocyclone during high-solid sludge anaerobic digester supernatant deammonification, and information for reducing particle breakdown, pumping selection, and separation optimization.

Determination of long-chain fatty acids in anaerobic digester supernatant and olive mill wastewater exploiting an in-syringe dispersive liquid-liquid microextraction and derivatization-free GC-MS method.

Long-chain fatty acids (LCFA) are commonly found in lipid-rich wastewaters and are a key factor to monitor the anaerobic digesters. A new simple, fast, precise, and suitable method for routine analysis of LCFA is proposed. The system involves in-syringe-magnetic stirring-assisted dispersive liquid-liquid microextraction (DLLME) prior to gas chromatography-mass spectrometry (GC-MS) without a derivatization process. Calibration curves were prepared in an ethanol solution (R2 ≥ 0.996), which was also useful as disperser solvent. Hexane was chosen as the extraction solvent. Several parameters (pH, ionic strength, extraction solvent volume, stirring time) were optimized in multivariate and univariate studies. Limits of detection (LODs) were found in the range 0.01-0.05mgL-1 and good precision inter-day (RSDs≤7.9%) and intra-day (RSDs≤4.4%) were obtained. The method was applied to quantify LCFA in supernatants of anaerobic digesters and olive mill wastewaters (OMW). Palmitic, stearic, and oleic acids were the most abundant fatty acid in the analyzed samples and the relative recoveries for all of them were between 81 and 113%.

Bioconversion of wastewater to single cell protein by methanotrophic bacteria

Single cell protein (SCP) provides an alternative protein source to partially replace the conventional agricultural resources and support the increased nutritional needs. Inexpensive feeding source is one of the key limiting factors for the expansion of SCP production. The present study examined the valorization of biogas derived from the anaerobic digestion (AD) of sewage sludge and the discarded effluent as nutrients source to produce SCP using methanotrophic bacteria. Results indicated that the mixed methanotrophic culture can grow well on the pasteurized AD supernatant and biogas, succeeding in promising dry weight (DW) yield (0.66 ± 0.01 g-DW/g-CH4 and 11.54 ± 0.12 g-DW/g-NH4+). Methylomonas (56.26%) and Methylophilus (24.60%) spp. were the two main representatives of the mixed culture. The produced dried biomass had a protein content higher than 41% w/w, including essential amino acids like histidine, valine, phenylalanine, isoleucine, leucine, threonine and lysine. The cultivated SCP shows potential utilization as protein source for animal diets.

Investigation of the impact of hydrodynamic parameters for phosphorus recovery from synthetic anaerobic digester supernatant in a fluidized bed reactor

Recovering phosphorus (P) from wastewater as struvite has been investigated widely as this approach represents a solution to several wastewater-related problems. Some of the typical process units applied in the industry for struvite recovery from wastewater streams include fluidized bed reactors where individual struvite crystals are being agglomerated into spherical granules, or pellets. Various hydrodynamic parameters affect the extent of inter-particle collisions, the intensity of mixing, localized concentrations of ionic species, nature of eddies, etc. which determine the efficiency of the crystal agglomeration. This study evaluated the impact of three key hydrodynamic parameters (up-flow velocity, feed injection nozzle velocity, and nozzle configuration) on P-removal and recovery efficiencies in a newly designed novel fluidized bed reactor (FBR). Lowering up-flow velocities from 60 cm/min to 20 cm/min resulted in higher P-removal and recovery efficiency, whereas high up-flow velocities were favorable for the production of larger pellets and fines. Nevertheless, higher nozzle velocity (18.04 m/s) was responsible for improving P-recovery efficiencies. The nozzle velocity had a strong influence on the morphology of pellets and fines. High nozzle velocity contributed to the formation of the smooth-surfaced pellets and plate-shaped fines. Three different nozzle-configurations generating different flow patterns within the agglomeration zone were studied.

Anaerobic Co-Digestion Effluent as Substrate for Chlorella vulgaris and Scenedesmus obliquus Cultivation

Anaerobic digestate supernatant can be used as a nutrient source for microalgae cultivation, thus integrating phytoremediation processes with high value products storage in microalgae biomass. Microalgae are able to use nitrogen and phosphorous from digestate, but high nutrient concentration can cause growth inhibition. In this study, two microalgae strains (C. vulgaris and S. obliquus) were cultivated on the anaerobic co-digestion supernatant (obtained from the organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS)) in a preliminary Petri plate screening at different dilutions (1:10 and 1:5) using a synthetic medium (ISO) and tap water (TW). Direct Nile red screening was applied on colonies to preliminarily identify hydrophobic compound storage and then a batch test was performed (without air insufflation). Results show that C. vulgaris was able to grow on digestate supernatant 1:5 diluted, while Nile red screening allowed the preliminary detection of hydrophobic compound storage in colonies. The analysis carried out at the end of the test on ammonia, phosphate, nitrate and sulphate showed a removal percentage of 47.5 ± 0.8%, 65.0 ± 6.0%, 95.0 ± 3.0% and 99.5 ± 0.1%, respectively.

Nitrogen recovery using a membrane contactor: Modelling nitrogen and pH evolution

A hollow fibre membrane contactor has been applied for nitrogen recovery from anaerobic digestion supernatant at different operating conditions obtaining nitrogen recovery efficiencies over 99 %. A mathematical model able to represent the time evolution of pH and nitrogen concentration during the recovery process is presented in this paper. The developed model accurately reproduced the results obtained in 26 experiments carried out at different pH values (from 9 to 11), temperatures (from 25 to 35 °C), membrane surfaces (from 1.2 to 2.4 m2) and feed flow rates (from 0.33 × 10−5 to 5.83 × 10-5 m3/s) predicting the variations in nitrogen recovery rates measured at the different operating conditions evaluated. Furthermore, due to the combination of nitrogen and pH modelling, the model is able to predict the variations in OH- concentration (alkali addition) required to increase and maintain the pH during the process. Thus, this model is a useful tool for process design and optimisation since it can predict nitrogen recovery rates and reagents consumption at different operational conditions such as flow rate, pH, membrane surface and temperature.

Membrane Filtration of Effluent from a One-Stage Bioreactor Treating Anaerobic Digester Supernatant

A challenge in side-stream treatment of anaerobic digester supernatant is that the effluent does not meet discharge standards. To address this challenge, this study tested tubular multichannel ceramic microfiltration (MF) and ultrafiltration (UF) membranes for the post-treatment of anaerobic digester supernatant. Pollutant rejection (total suspended solids (TSS), COD, total nitrogen (TN), and total phosphorus (TP)), color removal, and membrane susceptibility to fouling were determined at various transmembrane pressures (TMPs) (0.2, 0.3, 0.4, 0.5 MPa). Both methods completely removed TSS. In MF, COD was removed with 48–76% efficiency at 0.2–0.4 MPa. In UF, COD removal efficiency was slightly higher, reaching 83.7% at 0.4 MPa. With both methods, pollutant removal did not increase at TMP of 0.5 MPa. With both MF and UF, color was reduced by 54–100%, irrespective of the TMP. At 0.2–0.4 MPa, membrane resistance was lower and permeate flux was much higher with MF than UF. At 0.5 MPa, the methods differed only slightly from each other. Due to the larger cut-off, flux decline was slower in MF (0.7 h−1) than in UF (1.1 h−1), as the larger pore-size favors less foulant deposition. Thus, taking into account rejection efficiency, capacity, washing frequency, and cost (pressure), these results indicate that MF at 0.4 MPa is the most effective variant for post-treatment of anaerobic digester supernatant. With this variant, the almost colorless permeate contained 25 mg COD/L, no TSS, 55 mg TN/L (75% in the form of nitrites and nitrates), and 8.5 mg TP/L, thus meeting criteria for water to be used in irrigation or algae cultivation.

Study on the application of nitritation, treating different wastewater streams with high nitrogen content

As water pollution associated with nitrogen and its compounds become more epidemic, the seriousness of water pollution resulting from excessive amount of nitrogen becomes magnified. A number of studies were conducted to reduce nitrogen content in a variety of wastewater streams. In particular, the study of nitritation as an innovative method of nitrogen removal method had a relatively high attention because of its cost-effective benefit and environmentally benign process as proven. However, the research and development on the applicability and the operating efficiency of the nitritation has not been well known. In addition, nitritation was not applied to feasibly treat different wastewaters with variable contents of nitrogen. Accordingly, the current study was designed to analyze the characteristics of various wastewaters such as domestic sewage, primary clarifier supernatant, sludge thickener supernatant, anaerobic digester supernatant, sludge decanted water, and livestock wastewater. The results exhibited that, by strength, domestic sewage and primary clarifier supernatant were categorized as relatively low concentration wastewater, sludge decanted water as medium strength, and sludge thickener supernatant, anaerobic digester supernatant, and livestock wastewater as relatively high concentration wastewater, which showed very high nitrogen content. Therefore, such high nitrogen content will negatively impact on existing wastewater treatment process and its operating procedure. It was found that nitritation well proceeded with the high removal of ammonium nitrogen and best suitable to treat anaerobic digester supernatant with the highest concentration of ammonium nitrogen.

Arsenic removal and biomass reduction of As-hyperaccumulator Pteris vittata: Coupling ethanol extraction with anaerobic digestion

Improper disposal of arsenic-rich biomass and the lack of efficient methods to treat it may cause contamination in the environment. We developed an efficient method for arsenic (As) removal and biomass reduction of As-rich biomass of the As-hyperaccumulator Pteris vittata by coupling ethanol extraction with anaerobic digestion. This study assessed As partitioning among the three phases (gas, liquid and solid) after anaerobic digestion of P. vittata biomass. Biomass with and without As was first extracted with ethanol. Ethanol extraction removed ~93% As, with remaining As concentration at 197 mg kg−1. The extracted biomass was then digested at 35 °C under anaerobic conditions for 35 d. Arsenic in the digested biomass was reduced by 89%, with remaining As concentration at 60 mg kg−1. In addition, anaerobic digestion reduced the biomass by 64–71% and decreased the volatile solids content from 94 to 15–18%. Methane production was 145 and 160 LNCH4/kgVS after 35 d for As-rich and control biomass, respectively. As a final step, As concentration in anaerobic digestate supernatant was reduced to 0.26 mg L−1 by As-Mg precipitation. Overall, coupling ethanol extraction with anaerobic digestion decreased As concentration in P. vittata biomass from 2665 to 60 mg kg−1, or by 98%. At this level (<100 mg As kg−1), P. vittata biomass can be considered a safe material based on USEPA regulations. Effective As removal from P. vittata biomass prior to disposal improves the phytoremediation process and lowers biomass transport and landfill disposal costs.

Effects of pH, soluble organic materials, and hydraulic loading rates on orthophosphate recovery from organic wastes using ion exchange

Phosphorus recovery from organic wastes attracts more and more attentions because of the phosphorus sustainability challenges. Ion exchange (IX) was used in this study to recover orthophosphate (ortho-P) from the anaerobic digester (ADs) supernatant of an organic waste. The effects of pH, soluble organic materials (SOM), and hydraulic loading rates (HLR) on phosphate recovery were investigated by column tests. The results of tests with model water showed that the IX capacities were 64.2 mg P/g and 36.7 mg P/g at pH of 4.5 and 9.0, respectively. The desired HLR of ortho-P capture was 0.69 m/H (3 BV/h), with ortho-P capture efficiency and recovery efficiency of about 90% and 80%, respectively. The ortho-P desorption efficiencies with NaOH and KNO3 brines as eluents were investigated, finding that both NaOH and KNO3 could be substitutions of officially recommended NaCl to reduce the harmful effect on soil or plants in applications. The results of tests with ADs supernatant showed that the ortho-P capture capacity decreased to 14.3 mg P/g, with great disruption of SOM. Both ortho-P and SOM were desorbed well with over 85% ortho-P recovery efficiency and no detrimental effect on resin reuse. Regenerate with KNO3 could create mobile nutrients solution containing a N/ortho-P/K mass ratio of 17:8:75, which could balance deficient fertilizer. This research promotes the current understanding of phosphate recovery from organic wastes for fertilizer application.

Biochar seeding promotes struvite formation, but accelerates heavy metal accumulation

This study investigated the effects of biochar seeding (wheat straw biochar and rice husk biochar) on nutrient recovery via struvite formation, and improvements in the particle size of precipitated struvite from anaerobic digestate supernatant. Simultaneously, the influence of biochar seeding on heavy metal accumulation and elimination of pathogens (total coliforms and Escherichia coli) was evaluated under various operational factors, e.g., pH, supersaturation, reaction time, and seeding rates. Compared to the non-seeding process (maximum recovery efficiency of phosphate and ammonium 91% and 83%, respectively, with a particle size of 70 μm) and the struvite-seeding process (maximum recovery efficiency of phosphate and ammonium 97% and 94%, respectively, with a particle size of 100 μm), the process of biochar seeding improved nutrient recovery up to 7% and 11% for phosphate and ammonium, respectively, and increased struvite particle size by 43%, regardless of biochar type. XRD diffraction and FTIR analysis confirmed the prevalence of orthorhombic characteristics and an inner crystalline structure of the struvite formed by biochar seeding. About 75% of total coliforms and 70% of Escherichia coli were removed from the digestate supernatant through seeded struvite precipitation, regardless of the seeding materials. However, the biochar seeding process led to an accumulation of heavy metals in the acquired struvite product than that with non-seeded precipitation process. The concentrations of these metals were still well below permissible limits for application on agricultural land. It can be concluded that the inclusion of biochar as a seeding material might be a sustainable strategy to enhance struvite formation, intensify nutrient recovery, and yield high-quality struvite fertilizer with increased particle sizes.

Coupling effects of pH and Mg/P ratio on P recovery from anaerobic digester supernatant by struvite formation

Abstract Phosphorus (P) in wastewater may be recovered by the formation of struvite as a potential fertilizer. The pH of the solution and the dosage of magnesium (Mg) are two important control parameters for struvite crystallization and, in general, the addition of alkaline solutions to lower the pH values required for crystal formation usually represents a large percentage of the cost. Batch experiments showed that the initial pH of the solution had a greater effect on P recovery, pH change, and crystal size than the Mg dose. The largest average crystal size was observed at a pH of 8.4. A higher Mg/P ratio had a greater effect on the elimination of P when operation at lower pH, but these conditions resulted in higher purity of the struvite crystals. Moreover, a continuous experiment was carried out to demonstrate the synergistic effects of pH and Mg/P ratio in an air-agitated reactor. A comparative efficiency of P recovery (90.2–90.7%) for ranges of pH = 8.6 and Mg/P = 1.5 and pH = 8.4 and Mg/P = 2.0 was observed after 15 hydraulic retention times of operation. This result indicates that a satisfactory P recovery efficiency could be obtained at lower pH accompanied by a higher Mg/P ratio, which would lead to a decrease in the chemical cost of P recovery.

Impact of supersaturation ratio on phosphorus recovery from synthetic anaerobic digester supernatant through a struvite crystallization fluidized bed reactor

ABSTRACTOver the past few decades, several technologies have been developed to recover phosphorus (P) as struvite from wastewater. Although these technologies have achieved reasonable P-removal efficiencies, these technologies are associated with several shortcomings such as high capital and operating costs, longer crystallization time and production of low-quality product. This study focussed on the development of an efficient technology by designing a new fluidized bed reactor (FBR) and determining its optimum operating conditions. The supersaturation ratio is the most important process parameter for struvite recovery. This study exerted effort to establish a range of supersaturation ratios in order to achieve optimum P-removal and recovery with a lesser amount of fine crystals produced. Bench-scale FBR used in this study was able to accomplish 90% P-removal with 18% P-recovery. P-removal efficiency was observed to be increasing with an increase in the initial supersaturation ratio up to a value of 6.5. On the other hand, an increase in the supersaturation ratio resulted in a lower P-recovery efficiency with an increase in fines production. The supersaturation ratio from 5.5 to 6.0 was found to be optimum for efficient operation of the reactor.

Nutrient recovery from wastewater through pilot scale electrodialysis

Nutrient recovery performance utilising an electrodialysis (ED) process was quantified in a 30-cell pair pilot reactor with a 7.2 m2 effective membrane area, utilising domestic anaerobic digester supernatant, which had been passed through a centrifuge as a feed source (centrate). A concentrated product (NH4-N 7100 ± 300 mg/L and K 2490 ± 40 mg/L) could be achieved by concentrating nutrient ions from the centrate wastewater dilute feed stream to the product stream using the ED process. The average total current efficiency for all major cations over the experimental period was 76 ± 2% (NH4-N transport 40%, K transport 14%). The electrode power consumption was 4.9 ± 1.5 kWh/kgN, averaged across the three replicate trials. This value is lower than competing technologies for NH4-N removal and production, and far lower than previous ED lab trials, demonstrating the importance of pilot testing. No significant variation in starting flux densities and cell resistance voltage for subsequent replicate treatments indicated effective cleaning procedures and operational sustainability at treatment durations of several days. This study demonstrates that ED is an economically promising technology for the recovery of nutrients from wastewater.